Add support for categorical data

[dmey]

We can treat categorical data as discrete but first we need to pre-process categorical values by one hot encoding to remove the order. Re API we can change the current version from

# Assuming  an xarray datasets ds with X1 discrete and and X2 categorical 
generator.fit(ds, copula=syn.VineCopula(controls=ctrl), is_discrete={'X1': True, 'X2': False})

to something like

with X3 continuous 
g.fit(ds, copula=syn.VineCopula(controls=ctrl), types={'X1': 'disc', 'X2': 'cat', 'X3': 'cont'})